Start by purchasing a moisture meter at your local garden center. Use the moisture meter to check the soil moisture in your container at three locations around the tree rose to get a good reading, wiping the probe off prior to each testing. Then water when the soil is getting dry but do not keep it on the wet side. You might want to transplant the tree rose into a larger
container eventually, but the one you bought it in should be okay for about half the season. Check out this article for some additional information: https://www.gardeningknowhow.com/ornamental/flowers/roses/learn-more-about-tree-roses.htm

One other thing, be sure the container your tree rose is in has a drainage hole or holes. If it does not, then transplant it into a pot that does as soon as possible. No drainage hole or holes spells root rot problems.